The present invention relates to a method for the detection and identification of particles in a suspension, comprising the following steps:
a. generation of acoustic signals having the form of a beam using an acoustic source;
b. directing the acoustic signals at at least one measurement volume within the suspension, the boundaries of the measurement volume in the axial direction with respect to the acoustic source being defined with the aid of time windows;
c. reception of acoustic reflection signals produced by reflection of the acoustic signals by the particles in the at least one measurement volume;
d. conversion of the acoustic reflection signals into electrical reflection signals;
e. counting numbers of electrical reflection signals which have an amplitude in excess of a predetermined value and conversion thereof into numbers of particles which are larger than a certain size.
A method of this type is disclosed in British Patent 1,012,010, which describes a method and equipment for counting and measuring such particles, wherein acoustic samples are taken in various measurement volumes along the acoustic axis of the acoustic transducer in the suspension. By using suitable time windows when receiving reflected acoustic signals, the particles in, for example, four predetermined measurement volumes, which are each located a predetermined distance away from the transducer, are counted. By making use of a threshold voltage which the electrical signals produced from the acoustic signals must exceed in order to be counted, which threshold voltage is different for each zone, a minimum size for the particles to be counted is selected for each zone. Assuming that the particle distribution is the same in each zone, a rough estimate of the number of particles, subdivided according to particle size, can be obtained using this known method and using simple mathematical methods.
U.S. Pat. No. 3,774,717 describes a method and equipment for the detection and identification of small particles, for example biological cells, which, for example, are located in a medium which flows transversely to the direction of propagation of an acoustic signal. Each of the particles gives a specific scatter of the acoustic signal, depending on the size, the shape and the acoustic impedance of the particles. The acoustic signal has a wavelength and an effective cross-section of the order of magnitude of the particles to be detected and identified. Blood cells, for example, are detected with the aid of an acoustic signal of 860 MHz. Therefore, particles can be identified with the aid of techniques which are known from radar technology. The technique disclosed in this patent is unsuitable for in vivo detection and identification because the wavelengths used allow only very restricted depths of penetration in biological tissue.
The methods described above are based on the ultrasonic pulse-echo technique. With this technique use can be made of a so-called ultrasonic transducer, which converts an applied electrical pulse into an acoustic (ultrasonic) signal and which is also capable of converting an acoustic (ultrasonic) signal which is incident on the reception surface back into an electrical signal. Therefore, the transducer serves as transmitter and as receiver for ultrasonic signals. It is also possible to use independent transmitters and receivers. A high-frequency pulse-echo recording of the flowing suspension is made. A focusing transducer can be used for this. In FIG. 1 the "illuminating" sound signal is shown diagrammatically in an arrangement known per se.
As is shown in FIG. 1, the principal axis z of the sound beam 20 generated by a transducer 23 is perpendicular to the direction of flow P of the suspension 21 flowing in a channel 24 and, consequently, to the direction of movement of the particles 22 present in the suspension. When a particle 22 passes through the sound beam 20, the incident sound field will be reflected by the particle and the reflected signal will be captured by the transducer 23. The received signal is converted by the transducer 23 into an electrical signal, which is transmitted to the transmission and reception electronics 25. The transmission and reception electronics 25 transmit the signal to a computer 26, which is connected to a memory 27 for storing measurement data. The computer 26 is provided with suitable software for evaluation of the measurement data. The electrical signal from a single measurement is indicated diagrammatically by s and is a time signal, the time axis t indicating the propagation time of the sound. The response of the particle 22 in the measurement volume can be detected in the recording at that moment in time which corresponds to the propagation time of the ultrasonic pulse between transducer 23 and reflecting particle 22 and back again.
Only reflections within an applied time window [t.sub.1, t.sub.2 ] (see FIG. 1) are processed in the analysis. The measurement volume is thus limited in the axial direction by z.sub.1 =t.sub.1.c/2 and z.sub.2 =t.sub.2.c/2 (c is the speed of propagation of the sound). In the lateral direction the measurement volume is limited by the shape of the acoustic beam.
A method for counting the number of particles using a measurement set-up of this type is described in the above-mentioned patents and in Croetsch, J. G.; "Theory and application of acoustic particle monitoring systems", Jr. Advances in Instrumentation and Control 45 (1990), Part 1. Generally speaking, with this method a specific threshold value is chosen for the amplitude of the reflected signal. If the recorded signal within the time window [t.sub.1, t.sub.2 ] is in excess of this threshold value, this is interpreted as the presence of a particle in the measurement volume. On condition that the particle concentration is so low that the risk of the simultaneous presence of more than one particle within the measurement volume is negligible, the particle concentration can be estimated by counting the number of recordings in excess of the set threshold value.
This method takes no account of variations in particle size and no distinction is made between different types of particles which are possibly present in the suspension.